The mouse mammary tumor virus (MuMTV) causes mammary adenocarcinomas. This study proposes to investigate the numbers and locations of proviral copies of MuMTV in preneoplastic and neoplastic cells and relate this information to the carcinogenic effect of MuMTV. In addition, the transcriptional activity of the proviruses, as well as the possible role(s) of viral in RNA and proteins in carcinogenesis is under investigations.